Anatomia, desenvolvimento e morfologia polínica do androceu de Senna siamea (Lam.) H.S.Irwin & Barneby (Fabaceae)

Abstract

The morphology and diversity of stamens are important characters for distinguishing the genera Senna, Cassia, and Chamaecrista. However, there are still gaps in the knowledge of the anatomy, development, and possible functional differentiation of these stamens. Therefore, this study aimed to investigate the anatomy, development, and pollen morphology to assess whether there are structural indications of functional differentiation among the different stamens of Senna siamea (Lam.) H.S.Irwin & Barneby. To this end, flower buds and flowers at various developmental stages were collected in the municipality of Uberlândia, Minas Gerais, fixed in Trump’s fixative, and processed according to standard techniques in plant anatomy and palynology. The androecium of S. siamea has ten stamens organized in two whorls. The antesepalous whorl has two adaxial staminodes, two lateral-abaxial medium stamens, and one central abaxial long stamen. In the antepetalous whorl, there is one adaxial staminode, two adaxial medium stamens, and two abaxial long stamens. All fertile anthers are bithecous and tetrasporangiate. Anther wall development in the fertile anthers and staminodes is of the basic type. The fertile stamens have a persistent epidermis; the endothecium is lignified and varies in the number of layers among the stamens. The middle layers vary between stamens, and the tapetum is invasive nonsyncytial. In the staminodes, the tapetum remains undifferentiated. Regarding the number of endothecium layers, the anther of the long stamen of the antesepalous whorl, as well as the anthers of the medium stamens, have two layers, whereas the long stamens of the antepetalous whorl have three to four layers. The anthers of the long stamens have approximately seven middle layers, the anthers of the medium stamens have approximately five to six layers, and in the staminodes, these layers are initially undifferentiated. Pollen grain development occurs first in the anthers of the long stamens of the antepetalous whorl, whereas the staminodes do not produce pollen grains. Pollen grains are similar among the fertile stamens but differ mainly in the length and width of the ectoaperture. These results indicate that the external morphological differences among the stamens are also reflected in their internal structure and development; however, further analyses are needed to infer a possible functional division among the different stamen types.